Many petroleum field management situations require the knowledge of in-situ reservoir stresses and its evolution during the production life cycle of reservoirs. For example, in cases that there are pre-existing natural fracture networks, changes in the stress field may induce more or less production along some fracture directions, affecting the decisions on well placement and injection production strategies. Other important engineering considerations can be affected by compaction and subsidence in the reservoir and overburden. Geomechanical simulations are used to calculate the mechanical response of the reservoir rocks associated with fluids production or injection into underground formations. These simulations are used to make decisions during the reservoir management life-cycle. It is important to consider the effects of pre-existing fracture networks and faulting in the oil and gas production in conventional and unconventional reservoirs.
In order to simulate the geomechanical behavior of reservoirs, the modeling of both fluid flow and mechanical response are necessary to provide realistic results about the fully coupled behavior of reservoirs under production. The coupling is needed because the state of stress and deformation of the solid framework depend on the fluid pressure within the reservoir. This defines two different physical problems, in which both the physics of fluid and solid phases must be solved simultaneously. Therefore, the mathematical model to be solved is composed by two systems of partial differential equations governing the solid framework state of stress and pressure of the fluid and phases, respectively. The result of the fluid problem is used to solve the solid problem and, in its turn, the result of the solid problem is used to solve the fluid problem. After the partial differential equations of these problems are discretized, a numerical coupling scheme has to be used to enforce this interdependency. Known solutions use coupling schemes based on numerical convergence criteria only.
To solve the coupled fluid-mechanical system problem, different coupling strategies can be employed. The most commonly used schemes are: (1) fully coupling; (2) sequentially iterative coupling; (3) iterative coupling; (4) loosely coupling; (5) loosely staggered in time coupling scheme; and (6) explicit one way coupling. In the fully coupling scheme, the coupled governing equations of flow and geomechanics are solved simultaneously at every time step of the numerical solution. In the sequentially iterative coupling scheme, one problem is solved first, say fluid, and the other problem is solved using the intermediate solution result to iterate at every time step until the full solution converges. In the loosely coupling scheme, the two separate sets of equations are solved independently and information is passed at designated time intervals in both directions. In the loosely staggered in time coupling scheme, the total time step designated for the mechanics simulator is divided into multiple time steps for the flow simulator. At the end of the time interval the pore pressure is passed to the geomechanics simulator. In the explicit one way coupling, the two separate sets of equations are solved independently over the same total time interval and information is updated in only one direction, i.e., from flow simulator to the geomechanics simulator. Normally the points of coupling are based on convergence criteria.